Re: [patch 07/15] mm: memcontrol: fix excessive complexity in memory.stat reporting

[Johannes Weiner <hannes@cmpxchg.org>]

On Thu, Nov 30, 2017 at 02:15:31PM -0800, akpm@linux-foundation.org wrote:
> @@ -1858,9 +1824,44 @@ static void drain_all_stock(struct mem_c
>  static int memcg_hotplug_cpu_dead(unsigned int cpu)
>  {
>  	struct memcg_stock_pcp *stock;
> +	struct mem_cgroup *memcg;
>  
>  	stock = &per_cpu(memcg_stock, cpu);
>  	drain_stock(stock);
> +
> +	for_each_mem_cgroup(memcg) {
> +		int i;
> +
> +		for (i = 0; i < MEMCG_NR_STAT; i++) {
> +			int nid;
> +			long x;
> +
> +			x = __this_cpu_xchg(memcg->stat_cpu->count[i], 0);
> +			if (x)
> +				atomic_long_add(x, &memcg->stat[i]);
> +
> +			if (i >= NR_VM_NODE_STAT_ITEMS)
> +				continue;
> +
> +			for_each_node(nid) {
> +				struct mem_cgroup_per_node *pn;
> +
> +				pn = mem_cgroup_nodeinfo(memcg, nid);
> +				x = __this_cpu_xchg(pn->lruvec_stat_cpu->count[i], 0);
> +				if (x)
> +					atomic_long_add(x, &pn->lruvec_stat[i]);
> +			}
> +		}
> +
> +		for (i = 0; i < MEMCG_NR_EVENTS; i++) {
> +			long x;
> +
> +			x = __this_cpu_xchg(memcg->stat_cpu->events[i], 0);
> +			if (x)
> +				atomic_long_add(x, &memcg->events[i]);
> +		}
> +	}
> +
>  	return 0;
>  }

The memcg cpu_dead callback can be called early during startup
(CONFIG_DEBUG_HOTPLUG_CPU0) with preemption enabled, which triggers a
warning in its __this_cpu_xchg() calls. But CPU locality is always
guaranteed, which is the only thing we really care about here.

Using the preemption-safe this_cpu_xchg() addresses this problem.

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---

Andrew, can you please merge this fixlet into the original patch?

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 40d1ef65fbd2..e616c1b0e458 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1836,7 +1836,7 @@ static int memcg_hotplug_cpu_dead(unsigned int cpu)
 			int nid;
 			long x;
 
-			x = __this_cpu_xchg(memcg->stat_cpu->count[i], 0);
+			x = this_cpu_xchg(memcg->stat_cpu->count[i], 0);
 			if (x)
 				atomic_long_add(x, &memcg->stat[i]);
 
@@ -1847,7 +1847,7 @@ static int memcg_hotplug_cpu_dead(unsigned int cpu)
 				struct mem_cgroup_per_node *pn;
 
 				pn = mem_cgroup_nodeinfo(memcg, nid);
-				x = __this_cpu_xchg(pn->lruvec_stat_cpu->count[i], 0);
+				x = this_cpu_xchg(pn->lruvec_stat_cpu->count[i], 0);
 				if (x)
 					atomic_long_add(x, &pn->lruvec_stat[i]);
 			}
@@ -1856,7 +1856,7 @@ static int memcg_hotplug_cpu_dead(unsigned int cpu)
 		for (i = 0; i < MEMCG_NR_EVENTS; i++) {
 			long x;
 
-			x = __this_cpu_xchg(memcg->stat_cpu->events[i], 0);
+			x = this_cpu_xchg(memcg->stat_cpu->events[i], 0);
 			if (x)
 				atomic_long_add(x, &memcg->events[i]);
 		}

--
To unsubscribe, send a message with 'unsubscribe linux-mm' in
the body to majordomo@kvack.org.  For more info on Linux MM,
see: http://www.linux-mm.org/ .
Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>